1. Field of the Invention
This invention relates to brake valves for controlling fluid pressure for a brake system. More specifically, this invention relates to a differential master cylinder or two stage brake valve for supplying high volume and high pressure fluid inputs to a brake system.
2. Description of the Prior Art
The application of brakes in a motor vehicle can be viewed as a two stage process. The first stage is one of taking up the clearance between the brake facing and the disk or rotor element. In the next stage, the facing transmits braking pressure against the cooperating friction element. In the first stage, a high volume of fluid at relatively low pressure is required to quickly take up the clearance. In the following stage, the additional fluid volume demands are low, but pressure must be high to supply sufficient braking force. Therefore, it has become common practice to design brake valves with a large cylinder for supplying high fluid volume and small cylinder for supplying high fluid pressure. The large cylinder supplies a large volume of fluid without excessive pedal travel, while the small cylinder supplies high fluid pressure without excessive pedal force. It is also known to use a relief valve to relieve fluid pressure from the large diameter cylinder once a sufficient pressure rise is sensed in the brake supply lines. Pressure relief of the large cylindrical portion allows higher fluid pressure to be generated in the smaller cylinder section. Various forms of relief valves are known and are located either in the valve body or in the piston members. While it is preferable to locate the relief valve in the piston member and save space in the housing, locating the relief valve in the piston can increase the complexity or limit performance of the relief valve.
Differential master cylinders or two stage brake valves are also used in power braking systems that have manual operating capability. Therefore, it is desirable to have a brake valve arrangement wherein the large cylinder provides a high volume of fluid for the manual mode, but does not interfere with the operation of the brakes in the power mode.
In brake systems for industrial or agricultural equipment, it is also desirable to have independent braking of individual wheels. For example, on a tractor, it is common practice to have a separate brake pedal for each rear wheel. Providing each wheel with independent braking capability requires another brake control valve or master cylinder. Therefore, it is advantageous to minimize the space requirements of each cylinder, thereby reducing the total size of the brake housing. Moreover, vehicles having a differential in the power train are often provided with a differential lock for preventing wheel slip when additional traction is required. It is common practice to have means tied to the braking function for releasing the differential lock when the brakes are applied to either or both rear wheels. In the past, providing these various functions and capabilities has greatly increased the complexity and space requirements for the various control valves used in the brake system.